Axial Resolution in Ultrasound Imaging

• Resolution is the ability to accurately image
• Accuracy can be described in many ways and it comes in several types

Axial Resolution

• Measure of detail resolution in an image
• Measures a machine's ability to display two structures very close together
• This occurs when the structures are parallel to the main axis of the sound beam
• Deals with the minimum distance that two structures can be apart and be seen as two distinct structures on an ultrasound image
• Measured in mm
• Determined by the spatial pulse length
  • Shorter pulses improve axial resolution
• Can't be changed, as it is fixed to the spatial pulse length
• Typical value is 0.1 - 1 mm
  • Lower values equal shorter pulses and improved image accuracy
• Minimum reflector separation is required along the direction of sound travel to produce separate echos
• Lower values of axial resolution indicate a shorter pulse
• Shorter pulses create more accurate images
  • Therefore image quality is superior with lower numbers

Axial Resolution Formula

• Axial Resolution (mm) = SPL (mm)/2
• Axial Resolution (mm) = Wavelength (mm) x # cycles in pulse/2
• In soft tissue Axial Resolution (mm) 0.77 x # cycles in pulse/frequency
• Axial resolution is determined by the pulse length
  • Shorter pulses yield improved axial resolution

Creating a Short Pulse

• A short pulse can be created in two ways
  • Less ringing
  • Higher frequency which is a shorter wavelength
• A pulse is short if there are few cycles in the pulse
• Transducers create pulses that are only 2-3 cycles
• Dampening the crystal after excitation by an electrical signal reduces ringing
  • This keeps the transducer from ringing for a length of time
• Transducers are designed with backing material to have few cycles per pulse
  • This means that the numerical LARRD resolution is low and the image accuracy is superior
• A pulse is short if each cycle in the pulse has a short wavelength
• Shorter wavelengths are characteristic of high frequencies
• Pulses from high frequency transducers have superior axial resolution
• High frequencies improve image detail (resolution)
• Low frequencies provide deeper penetration
• Choose the transducer that is appropriate for your area of interest and imaging depth when using clinically
• Excellent axial resolution is associated with:
  • Shorter spatial pulse length
  • Shorter pulse duration
  • Higher Frequencies
  • Few cycles per pulse
  • Lower numerical values

Lateral Resolution

• Also known as:
  • Lateral
  • Angular
  • Transverse
  • Azimuthal
• Uses distances (mm, cm)
• Determined by the width of the sound beam, which varies with depth
• The minimum distance that two structures can be apart and still be seen as two structures when they are perpendicular to the sound beam
• Lateral Resolution = beam diameter
• Resolution of structures that are perpendicular to the beam
• Lateral resolution is best as the focus as the beam is narrowest
• The minimum distance that two structures can be apart and still be seen as two structures when they are perpendicular to the sound beam

Axial vs. Lateral Resolution

• Axial resolution is better because ultrasound pulses are shorter than they are wide, but both are important
• Higher frequencies improve both axial and lateral resolution
• Axial is improved because of the shorter pulses associated with high frequency sound, and is improved in the entire image
• Lateral is improved in the far field high frequencies diverge less than low frequencies, and is improved in the far field only because of less divergence

Estimating Lateral Resolution

• It needs to be known precisely what the structures that are being scanned look like
• Use a phantom and a tiny bb to measure it
• A structure would be measured that is a point reflector, which is a signle point and would appear sonographically as a wide bar.
• Measurement of the bar is the width of the sound beam, resulting in the lateral resolution

Focusing

• It concentrates energy in a sound beam
• Alters the beam in three ways:
  • Narrower "waist" in the beam
  • Shallower focus
  • Size of the focal zone reduced
• There are three methods of focusing:
  • Lens-external focusing
  • Internal focusing - curved crystal
  • Phased array focusing- electronic focusing
• Internal and external focusing can be used with single element transducers
• Phased array focusing is for transducers with multiple elements (arrays)

Types of Fixed Focusing Techniques

• Techniques of fixed focusing:
  • External focusing
    • Lens is placed in front of PZT
    • Similar to focusing light waves with a lens
    • Larger are in the lens creates more focusing
  • Internal Focusing
    • PZT is curved
    • Larger curve = more focusing
    • The most common form of fixed focusing

Electronic Focusing

• Includes phased array transducers
• Have better lateral resolution because the focus is adjustable by the sonographer
• System's electronics focus the sound beam
• Phased array technology provides dynamic, variable focusing, or multi-focusing

Focusing and Intensity

• Low intensity at transducer surface to create a very high site intensity at the focus point

Axial resolution measures an ultrasound machine's ability to display two structures very close together when parallel to the sound beam's main axis. It is determined by the spatial pulse length, where shorter pulses improve axial resolution. Typical values range from 0.1 to 1 mm; lower values indicate shorter pulses and superior image quality.